Prior Art telephone modems in general use, such as those constructed according to the V.34 or 56 K standard, typically use a symmetrical single-channel communications approach. Early telephone modems were designed for primitive computer systems (and other telecommunications applications) where it was presumed that the data flow between two computers (or computer and terminal) would largely be equal in both directions.
However, with the advent of Internet communications, the balance of data flow between a remote computer and a computer system has become severely lop-sided. In particular, in most internet communications, a remote user sends only a small amount of data "upstream" toward the computer system (e.g., ISP or the like) indicating his or her command and selection choices. In contrast, the amount of data downloaded (e.g., Web site graphical data, images, video, and the like) may be huge. Prior art modems design is based in the incorrect assumption that data flow will be approximately equal in both directions. Such a scenario does not make best use of available bandwidth in a telephone signal.
In addition, the single-channel approach of prior art modems does not begin to use all available bandwidth in a telephone line. Prior art modems were designed to operate over standard telephone lines for any given distance (including overseas and satellite calls) and thus were designed to perform well for the minimum amount of bandwidth in a telephone system. Most prior art modems (including V.34) are designed to operate within a narrow 34 KHz POTS (Plain Old Telephone Service) bandwidth.
Again, with the advent of Internet communications, modem usage has changed. Remote users are less likely to dial cross-country or across the world to connect with a computer system. Rather, a user will call a local Internet Service Provider (ISP) through a local telephone company central office (CO) and access remote computers via the Internet. The distance between a typical user and the nearest CO may be on the order of one to two miles, and thus the available bandwidth far in excess of the traditional 3.4 KHz POTS.
A new modem standard known as Asymmetric Digital Subscriber Line (ADSL) has been proposed and is already in limited use for applications such a Video-on-Demand and the like. ADSL Modems are described, for example, in American National Standards Institute standard ANSI T1.413-1995 entitled "Network and Customer Installation Interfaces--Asymmetric Digital Subscriber Line (ADSL) metallic interface", incorporated herein by reference.
In ADSL, the upstream and downstream data paths may not be symmetrical. The upstream data rate may be as high as 640 Kilo-bits per second (Kps) whereas the downstream data rate may be as high as 6 Mega-bits per second (Mps). ADSL also takes advantage of the increased bandwidth available in local phone service (as well as improved long distance bandwidth provided by fiber optics and the like) by using a plurality of discrete data channels to transmit data in both directions.
A description of ADSL architecture may also be found, for example, in ADSL, by Kimmo K. Saarela (Tampere University of Technology), incorporated herein by reference. In order to understand the present invention, a basic understanding of ADSL architecture and timing and synchronization controls is in order.
FIG. 1 is a frequency diagram illustrating the general bandwidth allocation for the ADSL modem specification. The lower 3.4 KHz is the traditional telephone bandwidth, otherwise known as "plain old telephone services" or POTS. Above that bandwidth, between approximately 30 and 138 KHz is allocated for the upstream channel. Note that the numbers illustrated in FIG. 1 are by way of example only.
The remainder of the bandwidth, up to 1104 KHz may be used for the downstream channel. Again, the numbers illustrated in FIG. 1 may be approximate.
FIG. 5 is a block diagram illustrating an overview of an ADSL modem system. A digital network (e.g., Internet, Internet Service Provider, on-line database, video-on-demand, or the like) may interface with central office ADSL transceiver unit ATU-C 520 through logical interface V. The output of ATU-C 520 may be mixed in splitter 550 with the output of public switched telephone network (PSTN) 570 and transmitted over loop interface 590 (e.g., copper twisted pair).
Splitter 560 may receive the combined signal and output the plain old telephone signal (POTS) otherwise known as message telecommunications service (MTS) 580. Digital data is output from splitter 560 to ADSL transceiver unit 530 which is interfaced through interface Tsm to customer installation 540 (e.g., bus star or the like) to service modules SM 544 and 542 (e.g., computer or the like). Data may also be transmitted upstream, from ATU-R 530 to ATU-C 520, as discussed above.
FIG. 2 is a frequency domain diagram illustrating how either of the upstream or downstream channels may be sub-divided into a number of discrete data channels each occupying 4 KHz bandwidth. For the sake of brevity, all of the 4 KHz channels are not illustrated in FIG. 2, which uses, by way of example, the downstream channel comprising 256 channels of 4 KHz each. Upstream communication comprises a total of 32 channels of 4 KHz each. Each channel may simultaneously transmit (with other channels) a number of bits, depending upon the type of modulation used (e.g, quadrature amplitude modulation or the like).
Each channel may transmit a fixed number of bits, from 0 to 15 every symbol time. However, the average number of bits per channel is approximately six bits. For the downstream path, six bits per channel, with 256 channels at 4 KHz results in a data rate of approximately six Megabits per second, a considerable improvement over the 56 K modems typically in use.
One of the channels illustrated in FIG. 2 (e.g., Carrier #64, f=276 kHz) may generate a pilot tone signal for synchronizing clocks between a downstream transmitter and corresponding receiver. For the sake of brevity, further discussion will be with regard to the downstream path, however it should be understood that the following information will also apply to the upstream path as well.
The synchronizing clock signal may be used in synchronizing the clocks of downstream transmitter and receiver upon initial connection, as well as after any micro-interruption or other disturbance where signal may be momentarily lost. In order to communicate data, the system must be able to synchronize properly after such a disturbance, and the ADSL modem specification specifies a frame symbol for this purpose.
As noted above, FIG. 2 illustrates the frequency domain. When converted to the time domain using an inverse Fast Fourier Transform (FFT), the 256 channels at one point in time generate 512 time domain samples. For practical purposes, 32 samples are added to the 512 samples to form cyclic prefix patterns. FIG. 3 is a simple diagram illustrating a group of 512 samples plus cyclic prefix patterns. Such a group may be referred to as a symbol.
The diagram of FIG. 4 is schematic only, to further illustrate the arrangement of data symbols. A group of 68 symbols may be concatenated (e.g., placed end-to-end) to form a superframe of data. With each superframe of data, one symbol is included as the synchronization symbol.
FIG. 4 illustrates the construction of such a superframe. FIG. 4 illustrates how the synchronization symbol (or frame) is transmitted every 69th symbol. Unlike the clock synchronization channel discussed above, which merely synchronizes clock signals, the synchronization symbol helps each modem to establish boundaries of groups of data, namely each symbol.
At a receive end when symbol synchronization is lost, the starting position of a subsequent synchronization symbol may be displayed from the position it would have been by some number of samples. This displacement may be referred to as symbol offset or offset.